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Put a plate in the freezer for you to test your jam is set later. Sterilise two 500ml jam jars so they are ready to use.
Put the blueberries, sugar, vanilla and lemon in a large heavy-based saucepan over a medium heat. Crush the blueberries a little with a potato masher. Bring to the boil, then reduce to a simmer and cook for 30 mins. Skim off any scum that has formed on the top with a spoon and discard.
Test the jam is set by spooning a mound onto your cooled plate. Put into the fridge for 3 mins. If the jam forms a skin and ripples when you push it with your finger, it's ready. If not, return to the heat for 5 more mins then repeat the test. Do this until you reach setting point.
Leave the jam to sit for 10 mins off the heat. Ladle into your warm, sterilised jars and seal the lids on top. Leave to cool completely. Will keep for six months unopened in a cool, dry place, or two weeks once opened in the fridge.